This invention relates to power supply circuits, and more particularly, to a low power transformerless, inductorless power supply circuit of the switching type.
One common form of power supply, which can be found in certain TV receivers, consists of a full wave bridge rectifier,a load capacitor (e.g. 330 .mu.F), and a linear regulator. The bridge rectifier generates a voltage which is filtered by the load capacitor. The load capacitor voltage is then fed to the linear regulator which in turn regulates the voltage at a preselected level, for example, 130 V.
This power supply has certain disadvantages and limitations. For a 120 V AC input, 170 V is generated across the load capacitor of the bridge rectifier. Since the linear regulator must produce an output voltage of 130 V, there is a 40 V drop across the regulator itself, drawing up to 1 amp of current. In order to protect the regulator from damage due to the excessive amount of dissipated power, a number of thermal resistors are connected in parallel with the regulator so as to shunt a part of the current. These resistors are bulky and thus are undesirable as they increase the cost of the overall system and furthermore occupy a significant amount of space on the TV board. In addition, they allow only a small variation in the line voltage because if the voltage rating of the load capacitor is exceeded, damage to the load capacitor as well as the linear regulator may occur. This power supply therefore requires a well regulated 120 V commercial line voltage. Since the load capacitor is charged only once for each half cycle of the AC supply voltage, higher peak capacitor charge currents result, along with other attendant difficulties.
U.S. Pat. No. 4,685,046 (8/4/87) describes a transformerless, low voltage, direct current power supply of the switching type adapted to energize a low voltage direct current load. A full-wave bridge rectifier is coupled directly across a 115 V, 60 Hz alternating current line to provide a 120 Hz pulsating direct current. A pair of transistors, alternately switching on and off in complementary fashion, continuously applies to a low power load only a leading edge portion and a trailing edge portion of each of the 120 Hz direct current pulses. The resultant direct current voltage, e.g. 11.3 peak volts across the load, is thus substantially reduced to a desirable level as compared to the peak voltage (161 peak volts) of the pulsating direct current provided at the bridge output. In another embodiment, the function of the transistors is provided by a pair of operational amplifiers which control the switching of a field effect transistor in series with the load. The control function is provided by sensing the input voltage of the power supply circuit. This has the disadvantage that if the input changes too abruptly and the output is not able to respond immediately to the change, the exact value of the output voltage cannot be well controlled. Furthermore, because the circuit threshold level is set by means of a zener diode, this makes the circuit less flexible in the event that the threshold has to be adjusted.
Another transformerless power supply which also senses the input supply voltage to determine the output voltage is described in DE 3304759 (8/16/84). In this German patent, a first op-amp compares a first input voltage, supplied by a full wave bridge rectifier, with a DC reference voltage supplied by a zener diode and a resistor voltage divider and a second op-amp compares a second input voltage with the DC reference voltage. If either input voltage is lower than the reference voltage, a switching transistor in series with a load capacitor is turned on. The load capacitor is only charged when the rectified AC supply voltage is below the reference voltage. The control function is once again provided by sensing the input voltage. This circuit uses a high-side power switch which produces higher power losses when compared with a low-side switch and results in problems in the integration thereof.